This article: Introduction POLCA
|QRM: The cellular organization|
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POLCA was invented by the American professor Rajan Suri. He positions this work-load controlling system as part of a broader management philosophy, called Quick Response Manufacturing (QRM). In that approach, lead time reduction is the Holy Grail.
POLCA (introduction article)
POLCA: the (capacity-controlled) cellular factory
By Dr Jaap van Ede, editor-in-chief business-improvement.eu. The first version was published in a Dutch specialist journal,
PT Industrieel Management (may ‘07). Since then, the article has been regularly updated.
It's common knowledge that Lean Manufacturing aims at eliminating waste, such as (intermediate) stock. Thereto, production lines are formed which produce in flow, controlled with Kanban-cards.
Unfortunately the number of companies which make many different products is growing. In that situation, the Kanban system doesn’t work well, because for Kanban it is needed to keep dedicated intermediate stock for all products, see the box “Kanban versus POLCA”.
The common approach is then to re-organize the work floor by forming flexible workcells. Each of those cells is staffed with a multidisciplinary team, and each cell performs a group of similar tasks (such as the execution of similar manufacturing processes).
‘My thesis dealt with the question how to make the throughput time as small as possible, when you have a complex product mix’, says Riezebos. ‘It turned out to be necessary to split up the bill-of-material. Only then it is possible to control the supply per work cell with an ERP-system.’
But that was not enough!. ‘I found that there was a great need for a signaling system comparable with Kanban, to gear the activities of material-exchanging workcells to eachother’
KANBAN versus POLCA
When a company has a complex product mix, Polca proves to be superior to the Kanban-system.
To explain why, we take a printing business as an example. In this business, each order starts at a desk-top station.
As an example, we follow an order that must travel the route:
^Production control with a Kanban-loop. © C.J. van Ede & J. Riezebos (2007)
To this inventory a Kanban-card was connected. Station PE sends this card to the DE-station, this is step 2. The message is simple: “I consumed material #089, please send me new”. In step 3, station PE produces the prescribed amount of material on the card, and replenishes the stock with serial number #089 at station PE.
Thanks to the Kanban-system, the amount of stock per item remains (very) low. After all, replenishment only takes place after consumption.
The conclusion is, that Kanban doesn’t work well when the product mix is complex. But it becomes even worse: In the printing business, Kanban does not work at all! Because each product should be made conform the description by the customer, it is impossible to know a priori what intermediate stock is needed. In other words, every production step is unique and can only start after a printing order has arrived.
However, there is also a crucial difference. A Kanban card signals a supplying work station that there is need for replenishment of specific materials, while a POLCA-card signals to a supplying workstation that there is free capacity to work on semi-finished products, produced and to be delivered by that work station. Note that it doesn’t matter which products, since a POLCA card has no article number on it. In addition, with POLCA nothing is produced to stock.
To clarify this, let’s follow the order again that travels the route:
With POLCA, station DE first needs permission to work on this order. This permission is granted, if and only if the two conditions below are fulfilled:
A POLCA-card resembles a domino, with on one side the supplying work station, and on the other side the receiving work station. In this case station DE therefore needs a POLCA-card DE/PE to start with this specific order.
^ Production control with a POLCA-loop. © C.J. van Ede & J. Riezebos (2007)
Next the semi-finished product will, together with the DE/PE card, be handed over to station PE, to execute the second production step. However, station PE is only allowed to start with that, if there is free capacity on the next and in this case final work station A4, which is the four color printer. That will be the case if station PE has at least one POLCA-card PE/A4 (which gives permission for delivery from PE to A4).
So, to process the order coming in from DE on station PE, two POLCA-cards are needed, forming a ‘domino street’ DE/PE PE/A4 (see step 2). When station PE is finished working on the product, card DE/PE is returned to the supplying work station DE (step 3a). Card PE/A4, together with the semi-finished product, is send to work station A4 (step 3b).POLCA is not only the name of a dance, although - according to Rajan Suri - this system makes ‘materials dance across the shop floor’! POLCA is also an acronym for Paired-cell Overlapping Loops of Cards with Authorization.
‘Paired-cell' indicates that work stations are connected by circulating cards, to level the workload. ‘Overlapping’ refers to the fact that an intermediate work station needs two POLCA-cards to start production, as demonstrated above.
POLCA functions without intermediate stock, therefore this system works well in the printing business. In addition, POLCA prevents that a work station becomes overloaded with work-in-progress, because only new materials are sent if there is free capacity to process these further. If that is not the case, supplying work stations will work on orders for other receiving work stations.
This control system is comparable to traffic lights, which do give access to a quiet motorways, but block the entrance to (at that moment) heavily used roads. Only the entrance of production orders is allowed, for which there is enough capacity at work cells downstream. If this is not the case, cells will work on orders that follow a different, more 'quiet' route.
Not always possible
^ Parker Filtration investigated the feasibility of POLCA-control
Limitations of POLCA
POLCA is a production planning system for companies making a wide range of products, and/or making customer-specific products. An increasing number of companies meets this definition. However, the POLCA-system is not the ideal solution for all of them. Three boundary conditions have to be met:
If a shop floor meets all the conditions above, still implementation of POLCA is not always (fully) possible. As discussed in the case-study Parker Filtration, it can be a problem if there are many converging or diverging production routes.
An example may clarify this. Suppose that workcell V1 supplies semi-finished products to cell M1, that need processing at cell E1 ór E2 ór E3 later. In that case, the POLCA-system will not automatically distribute the product stream, via M1, evenly over those cells. It is possible to adjust the production plan on a higher level with an ERP-system, but this becomes unfeasible if there are a lot of those situations.Despite of all the above mentioned conditions and problems around POLCA, it remains worthwhile for high mix/low volume producing companies to study it. After all, it is not necessary to implement this system everywhere on your shop floor. So, perhaps, locally good results can be achieved.
1) According to Rajan Suri, splitting up the shop floor is not always needed to apply POLCA. Olson Engineering appears to have implemented POLCA-control without work cells.
Riezebos: ‘In short, the first step is to examine if the product mix fits. If that is the case, we investigate if the production can be organized in such a way, that applying POLCA becomes possible.’
In may 2007, there were still no companies in The Netherlands which had implemented POLCA. However, the tide seems to be turning. In 2008, Bosch Scharnieren, a producer of made-to-measure hinges, adopted the system successfully.
Though the POLCA-system has certain limitations, it definitely adds something new to the process improvement toolkit, says Riezebos. ‘Unfortunately there is a lack of documented cases. In America I attended a workshop with managers of John Deere. This manufacturer of tractors seems to apply POLCA, but there is no scientific report about their results.’
Riezebos thinks that for a major breakthrough a large company is needed, that can do the pioneering work. ‘Like what General Electric did for Six Sigma. POLCA is however a detail planning system, and that lies outside the environment percieved by topmanagers. That
Lean ánd Agile?
The contrast between efficiency and flexibility to produce conform customer wishes is also called the dilemma of Lean versus Agile. This theme returns regularly on this site. One of the possible solutions is the POLCA-system, as described on this page. Another solution is to stick to ‘common’ Lean manufacturing, but with the introduction of productions cells, in which product families are produced. An example of this strategy can be seen in the Dutch company Vlastuin. A third possibility is applying CONWIP, CONstant Work In Progress. This can be seen as a strongly simplified version of POLCA, because in that case cards only signal when there is room for extra work in progress on the shop floor.
‘QRM was developed before POLCA’, Suri points out. ‘But when I implemented QRM within companies that produce small volumes to order, I noticed how these companies were struggling with Kanban and production control via their ERP-system. To solve that problem, I invented POLCA.’
In addition, Suri points to several disadvantages of Lean Manufacturing. The strong focus on just-in-time supply will, in his opinion, lead to a shift of inventory to the suppliers. Further, reducing waste locally is not always good. Sometimes it can for example be profitable to reduce the product batches, though this raises the set-up-times. This apparent increased waste can be acceptable if the total lead time from raw materials to delivery is reduced. Therefore, QRM focuses on the elimination of waiting times, instead of reducing waste.
'The Polca-system helps to make a clear distinction between QRM, Lean and the TOC’, thinks Riezebos. Undoubtedly this is true. The development of the drum-buffer-rope principle laid the basis of the TOC. Later, this way of thinking was broadened to resolve bottlenecks in factories and supply chains. In a similar way, POLCA can be seen as one of the most important pillars of QRM. It is a new mode of thought, which can be widened to control supply chains.
Case: Parker Filtration
Frank te Hennepe is operations manager at Parker Filtration in Arnhem, The Netherlands. ‘In cooperation with the Dutch researchers Jan Riezebos en Jacob Pieffers we investigated if it was possible to implement POLCA. I remember their first reaction well: Wow, that’s not easy here!”
Te Hennepe paints a picture of the existing situation at Parker-Filtration: ‘The main-stream of our production, lets say 80% of the work, is controlled with Kanban. The production of customer-specific parts is tuned to that by human planning.’
Parker Filtration produces, among others, filters for hydraulic oil circuits. ‘80% of the parts needed to built those filters are standard. These parts are assembled via a production line which is made up of several production cells. This line is controlled with Kanban and produces in a One Piece Flow. Supply of customer-specific parts takes place crosswise to the central line.'
Also these customer-specific components are produced in work cells, but these are not controlled with Kanban. ‘We have two team leaders in our factory. Together with our ERP-system, they tune the production of the customer-specific parts to the rhythm of the One Piece Flow. In fact our team leaders do the same job as a POLCA-system would do, but their abilities go beyond that.’
^ Production at Parker Filtration: Kanban controls the production
of standard components via a central production line. Production
of customer-specific parts is tuned to that by human planners.
Replacing the team leaders with a POLCA-system turned out to be unfeasible. ‘Mixing Kanban and POLCA was not the bottleneck, there were other problems. First, the processing times per component are highly variable in our situation. Second, we often have converging or diverging production streams.’
And it becomes even more complex. ‘To assemble an oil filter, as a customer-specific part, you need a filter element that is folded during production. When you do that, the remainder of the material should be used to make other elements, whenever possible. Such things you can’t control with Polca-cards.’Parker Filtration however found another solution. Once again this is Kanban for the main stream, and human control for the remaining activities. ‘Production of common filter elements we control with Kanban. The material that remains can be used to make special types of elements.’
> Discouraged by this case? Perhaps no need to be. See, for a successful implementation of POLCA: case “BOSCH scharnieren”
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